The “Erk pathway” is an intracellular signal transduction pathway used by nearly all types of human cells to translate extracellular signals to cellular decisions, including proliferation, differentiation, senescence, or apoptosis (Wellbrock et al., Nat. Rev. Mol. Cell Biol. 11:875-885 (2004)). One of the invariant components of this pathway is the Ras GTPase, which receives signals from membrane receptors and activates the Raf protein kinases, which activate the Mek protein kinases, which in turn activate the Erk protein kinases. Activated Erk kinases phosphorylate a number of nuclear and cytoplasmic targets to initiate various cellular decisions. The biological importance of Raf in the Erk pathway is underscored by the finding that mutated forms of Raf are associated with certain human malignancies (see e.g. Monia et al., Nature Medicine 2:668-675 (1996); Davies et al., Nature 417:949-954 (2002)). Three distinct genes have been identified in mammals that encode Raf proteins; a-Raf, b-Raf and c-Raf (also known as Raf-1) and isoformic variants that result from differential splicing of mRNA are known (Chong et al., EMBO J. 20:3716-3727 (2001)). The Erk pathway is mutationally activated in a number of human cancers, most often by mutation of the Ras or b-Raf genes. Mutations in Ras and b-Raf genes generally occur in the same tumor types, including cancers of the colon, lung and pancreas and melanoma, but are usually mutually exclusive. This suggests that activation of either Ras or Raf is sufficient for pathway activation and cancer progression.
Since tumor cells frequently become dependent, or ‘addicted’ to one or two key signaling pathways for their survival (see, e.g. Jonkers et al., Cancer Cell. 6:535-538 (2004)), the Erk pathway represents a highly attractive target for drug intervention to treat cancer. Protein kinases in general are considered desirable targets for drug therapy, as evidenced by recent successes in targeting growth factor receptor and intracellular tyrosine kinases. Inhibitors of Mek have shown promise in clinical trials, however, there is ample evidence to indicate Mek-independent Raf signaling that may also contribute to cancer progression (Wellbrock et al, Nat. Rev. Mol. Cell Biol. 11:875-885 (2004)). Therefore, targeting Raf kinases promises an alternative and complementary approach to treating tumors in which Ras or Raf genes are mutated.